The recovery of uranium from wet-process phosphoric acid has been the subject of prior study. Most marine phosphate rock contains from about 0.2 to 0.4 pounds of uranium per ton. Thus, the annual production of phosphate rock, on the order of about 30-40 million tons yearly, represents several million pounds of uranium. In the United States, both the Florida and Carolina phosphate deposits contain appreciable uranium.
In the wet process for producing phosphoric acid, phosphate rock is treated with sulfuric acid, thereby precipitating calcium sulfate and releasing phosphoric acid. It has been found that nearly all of the uranium contained in the phosphate rock is dissolved by sulfuric acid, and that the filtrate of the crude phosphoric acid contains from about 70% to about 90% of the uranium in the original phosphate rock.
To recover the minute uranium content of the crude wet-process phosphoric acid required special processing techniques. These techniques are generally described in the Kirk-Othmer Encyclopedia of Chemical Technology, Second Edition, volume 21, pages 1 to 36, which are hereby incorporated by reference. One technique for recovering uranium from crude wet-process phosphoric acid is solvent extraction. Recovery of uranium from crude phosphoric acid by solvent extraction requires a solvent which is immiscible with water and which has a high affinity for uranium. These solvents extract uranium from crude phosphoric acid by the formation of a complex with the uranium which is soluble in excess solvent.
After separation of the uranium-laden or "pregnant" solvent from the phosphoric acid, the solvent is contacted with hydrofluoric acid to precipitate the uranium in the form of impure uranium tetrafluoride hydrate, UF.sub.4 .n H.sub.2 O, which is removed from the solvent-hydrofluoric acid mixture by means of a centrifuge. Because of its impurity content, the resulting UF.sub.4 .n H.sub.2 O is not suitable for direct conversion to UF.sub.6 and must be first purified to produce a uranium product meeting "yellow cake" standards.
Prior processes to purify uranium tetrafluoride hydrate, UF.sub.4 .n H.sub.2 O, include for example, digestion of the crude uranium tetrafluoride hydrate in hot concentrated sodium hydroxide solution, followed by filtration and washing. It has been found that in order to consistently meet the standards of purity required for the production of "yellow cake" a large excess of sodium hydroxide or multiple digestions are required to digest the uranium tetrafluoride in the initial step of the process. In addition, filtration of the solution after digestion in concentrated sodium hydroxide is difficult and requires considerable filter area. This increases the cost of equipment and the operating cost of the described process.
The present invention provides a new process of purifying uranium tetrafluoride hydrate to produce a uranium (VI) peroxide product meeting "yellow cake" specifications which is economically attractive, especially when compared to the prior art process described above. Yellow cake specifications are issued by uranium refineries and specify the maximum impurity levels which a refinery will accept in their refinery feedstock. For example, yellow cake specifications issued by the Kerr-McGee refinery specify a maximum fluoride level of 0.15% based on the weight of the uranium. Maximum levels of other impurities, based on the weight of uranium in the feedstock, include calcium, 1.00%.